The method and system disclosed herein, in general, relates to information communication. More particularly, the method and system disclosed herein relates to communicating distinct data over a single frequency using multiple signals under two different polarization schemes.
Current satellites and ground based radios, typically reuse frequencies by transmitting signals in two polarities of one of two polarization schemes: left and right hand circular polarization, or vertical and horizontal linear polarization. Normally, if different data signals are transmitted on the same frequency in both linear and circular polarizations, the data signals would interfere with each other. Consequently, no more than two different data signals are transmitted on the same frequency at an instant on the same or proximate path, representing the two polarities of a selected polarization scheme.
Another form of frequency reuse is through separation. Additional channels can be transmitted on the same frequencies as long as there is sufficient distance between the transmitters so that antennas can pick up the selected transmissions with minimal interference. In the case of satellites, the satellites must maintain a distance of about two degrees of arc before the same frequencies can be reused. In the case of broadcast television, terrestrial microwave radios, and for commercial radio stations geographic distance is used to ensure sufficient distance or attenuation between the transmitters.
Another form of reuse is similar to separation described above. Instead of physical separation the separation is caused by use of directional antennas. For example, one cell tower can hold several directional antennas each pointing in a different direction. Each antenna carries different data signals on the same frequencies. Because of the directionality of each antenna, only signals on the front side of the antenna can be received or picked up. This technique is often used in cell phone communications.
Two forms of polarization are referenced in this application, linear and circular polarizations. In linear polarization, the electric component or the magnetic component of an electromagnetic wave is confined to within a single plane along the direction of propagation of the electromagnetic wave. Linearly polarized signals are either horizontally polarized or vertically polarized, each being orthogonal to the other. In circular polarization, the tip of the electric field vector is made to describe a circle as time passes. Circularly polarized data signals are either right hand circularly polarized or left hand circularly polarized. Both linear and circular polarization schemes are the two extremes of elliptical polarization.
Polarization can be established by various methods, for example, through the shape of the radiation elements in the antenna in the case of a lower frequency antenna, for example a frequency modulated (FM) radio antenna, or by a feed horn often feeding a larger usually parabolic reflector in a higher frequency band. A circular polarized antenna might involve two linear dipoles orthogonal to each other with a 90 degree phase relationship between the radiated signals. The different polarization schemes and this disclosure apply to any frequency electromagnetic waves that can be polarized including, for example, light, microwave, and radio frequency waves.
A basic principle of electromagnetic waves is the principle of linear superposition: “when two or more waves are present simultaneously at the same place the resultant wave is the sum of the individual waves.”    Physics 3rd Edition by Cutnell/Johnson, Wiley and Sons, 1995. ISBN 0-471-59773-2, page 521.
As used herein, the term “feed horn” or “feed” refers to an apparatus that includes both a horn and a transducer, also called a polarizer. The transducer (OMT) radiates and polarizes the signal for transmission. The horn shapes the signal. A typical transducer is a mechanical device that bolts to the horn. The horn illuminates the antenna, as well as picks up already polarized data signals for reception. A transducer also routes the data signals from a transmission side of input flanges to the horn or from the horn to a reception side of output flanges. A transducer can form or receive orthogonal signals of linear, circular or both polarities. This is an example of one antenna design; however, this example is not meant to limit this disclosure to a particular antenna design.
As used herein, “data signal” refers to an electromagnetic signal modulated to carry information of any kind.
Electromagnetic waves do not interact when transmitted through a non absorbing media such as space. Left hand circularly polarized data signals and right hand circularly polarized data signals do not interfere with each other once transmitted. Similarly, horizontal and vertical linearly polarized data signals do not modify each other once transmitted and pass through space without interference. One of the characteristics of a linear receive antenna is that when aimed toward the circularly polarized signal source each pole of a linear feed and transducer, whether horizontal or vertical, picks up both left hand circularly polarized signals and right hand circularly polarized signals simultaneously in almost equal levels of about 3 decibels (dB) less than, therefore half of, the full strength of a correctly aligned circularly polarized feed. Similarly, when directed toward the source, an antenna or feed of circular polarity picks up any transmitted horizontal and vertical linearly polarized signals in both left and right circular polarities at about half the level of what a correctly aligned linear feed would. Along the axis of transmission, the rotation or angle of the linear receive feed in relation to the circular polarized transmission feed does not affect the reception level of circularly polarized signals in the linear feed. Similarly, each pole of a circular feed and transducer, left hand circular or right hand circular, picks up each horizontally polarized signal in almost equal levels and each vertically polarized signal in almost equal levels.
On any given frequency, normally attempting to transmit both linearly polarized signals and circularly polarized signals simultaneously results in so much interference in a receive antenna that the received signal is not usable. Linearly polarized signals suffer from interference from circularly polarized signals, whereas circularly polarized signals suffer from interference from linearly polarized signals.
A patent, U.S. Pat. No. 7,957,425, issued to this same applicant discloses a method to increase capacity in a radio frequency system by adding two inverted signals in a different polarization scheme from a first polarization scheme. This present disclosure shows a way to add a fourth unique data stream on the same path under certain conditions. In this disclosure two circularly polarized signals, left and right hand circular polarized, each carrying a unique data stream and are transmitted. Two inverse, 180 degrees out of phase to each other and equal amplitude, data signals carrying a third data signal are transmitted in linear polarities orthogonal to each other, for example horizontal and vertical linear signals. A fourth data signal is encoded onto a fourth carrier, two copies are made, then one is inverted (180 degrees out of phase) from the other. The fourth data signals are then transmitted orthogonally to each other and at 45 degree rotation around the transmit axis from the two linear signals carrying third data. This technique allows for transmission of up to four data signal along a same or proximate path on the same frequency at the same time. The previous patent, U.S. Pat. No. 7,597,425, and the discussion below discusses reception and decoding of the inverse linear data signals with the interfering circular data signals.
Another embodiment uses three pairs of inverse data signals. Two data signals and their inverses are transmitted in linear polarity and a third data signal and its inverse are transmitted in circular polarities.